Apparatus for smelting and reducing iron ores

ABSTRACT

An apparatus for smelting reduction comprising a preheat and prereduction furnace for preheating and prereducing iron ores; a smelting reduction furnace into which iron ores, carbonaceous material and fluxing material are charged and in which the iron ores are smelted and reduced; a top-blow oxygen lance through which oxygen gas is blown into the smelting reduction furnace, having decarbonizing nozzles and post combustion nozzles; and the smelting reduction furnace having bottom and side tuyeres built respectively in a side wall and a bottom thereof.

This is a division of application Ser. No. 07/246,456 filed Sept. 19,1988 now U.S. Pat. No. 4936908.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for smelting and reducing ironores and an apparatus therefor, and more particularly to a method forcharging and blowing in a smelting reduction process and an apparatuswith a constitution suitable for the method.

2. Description of the Prior Art

Smelting reduction method is to be substituted for a blast furnace ironmaking method and has recently been under research and development toovercome a disadvantages in that the blast furnace iron making methodrequires not only an expensive construction cost but also a vastconstruction site.

It is well known that in this smelting reduction method, prereduced ironores or iron ores, as material, are charged and furthermore carbonaceousmaterial, as fuel and reducing agent, and fluxing material such as limestone are charged onto a molten metal contained in a smelting reductionfurnace, while oxygen is blown in the smelting reduction furnace. Thus,the carbonaceous material is melted into molten metal and C contained inthe carbonaceous material is oxidated. CO gas generating from the moltenmetal are post-combusted into CO₂ gas by means of O₂ gas excessivelyblown in. Sensible heat of this CO₂ gas is transferred to slag coveringthe face of the molten metal and to iron grains suspended in the slagand then to the molten metal. In this way, heat necessary for reductionof iron ores are transferred to the iron ores to be reduced and themolten metal is efficiently produced.

However, when in order to reduce the weight of reduction of iron ores,prereduction ratio of the iron ores before transference thereof to thesmelting reduction furnace, exhaust gas circulated from the smeltingreduction furnace to a prereduction furnace is required to below-oxidated gas and be of a large amount. For this reason, thecombustion ratio of CO in the smelting reduction furnace is lowered andas a result, this prior art smelting reduction method is disadvantageousin that the heat efficiency ratio of the smelting reduction furnace islowered.

SUMMARY OF THE INVENTION

In the light of the mentioned difficulty, it is an object of the presentinvention to provide a method for smelting reduction and an apparatustherefor wherein the heat efficiency ratio is improved in a smeltingreduction furnace to promote smelting reduction without raising aprereduction ratio of iron ores in a prereduction furnace.

To attain the object, in accordance with the present invention, a methodis provided for smelting and reducing iron ores which comprises thesteps of:

the step of charging molten metal, carbonaceous material and fluxingmaterial into a smelting reduction furnace;

preheating end prereducing iron ores; charging the preheated andprereduced iron ores into the smelting reduction furnace;

the step of blowing oxygen gas into the smelting reduction furnacethrough an oxygen lance disposed in the top of the smelting reductionfurnace and having decarbonizing nozzles and post combustion nozzles;and

the step of blowing in stirring gas through side tuyeres built in a sidewall of a smelting reduction furnace and through bottom tuyeres built ina bottom of the smelting reduction furnace.

Furthermore, in accordance with the present invention, an apparatus forsmelting reduction fitted for the method comprising:

a preheat and prereduction furnace preheating and prereducing iron ores;

a smelting reduction furnace into which iron ores, carbonaceous materialand fluxing material are charged and in which the iron ores are smeltedand reduced;

a top-blow oxygen lance through which oxygen gas is blow in into thesmelting reduction furnace, having decarbonizing nozzles and postcombustion nozzles; and

the smelting reduction furnace having bottom tuyeres built respectivelyin a side wall and a bottom of the smelting reduction furnace.

The object and other objects and advantages of the present inventionwill become clear from the detailed description to follow, taken inconjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an embodiment of an apparatus forthe method of the present invention;

FIG. 2 is a view illustrating an enlarged tip of an oxygen lance of asmelting reduction furnace included in the apparatus for the method ofthe present invention;

FIG. 3 is a graphic representation showing a relation between Fe contentin slag (T, Fe) and flow amount of stirring gas blown in through bottomtuyeres built in a bottom of the smelting reduction furnace according tothe present invention;

FIG. 4 is a graphic representation showing a relation of a differencebetween temperatures of slag and molten metal with flow amount ofstirring gas blown in through side tuyeres built in a side wall of thesmelting reduction furnace according to the present invention;

FIG. 5 is a graphic representation showing a relation between anoxidation degree of exhaust gas generating from the smelting reductionfurnace and an injection angle for a post combustion nozzle according tothe present invention; and

FIG. 6 is a graphic representation showing a relation between a slagratio and basicity of slag according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Now referring specifically to the drawings, a preferred embodiment ofthe present invention will be described. FIG. 1 of the drawing shows ablock diagram of an embodiment of an apparatus used for a method forsmelting and reducing iron ores according to the present invention. In asmelting reduction furnace 10, molten metal bath 11 and slag layer 12are formed, first shoot 13 through which lime stone and flux is set inan upper portion of the smelting reduction furnace and oxygen lance 21through which oxygen gas is blown in is vertically inserted down intothe smelting reduction furnace.

FIG. 2 of the drawing illustrates an enlarged vertical section view ofthe tip of oxygen lance 21 used for an embodiment of a method forsmelting and reducing iron ores according to the present invention. Asseen from FIG. 2, decarbonizing nozzles 22 and post combustion nozzles23 are arranged in the oxygen lance 21 so as to supply oxygen gasindependently through the decarbonizing nozzles and the post combustionnozzles from individual oxygen supply sources set outside the smeltingreduction furnace by means of controlling a pressure and a flow amountrespectively.

Furthermore, center nozzle 24 for supplying mainly carbonaceousmaterial, lime stone or the like other than iron ores is arranged,passing through the center tip of the oxygen lance. The center nozzlecan be replaced by the decarbonizing nozzles, thereby to introduce thecarbonaceous material and the lime stone or the like together with theoxygen gas, depending on capacity and operational condition of thefurnace 11. Arrow symbols 28 and 29 at the tip of oxygen lance 21, each,show directions of injecting out oxygen gas respectively through thedecarbonizing nozzles and the post combustion nozzles. Chain line 40 isthe center axis of the oxygen lance.

Above said smelting reduction furnace 10, there is provided preheat andprereduction furnace 30, which is a fluid bed type reaction vessel,having second sheet 31 and third sheet 32. Through the second shoot 31,iron ores are charged into the preheat and prereduction furnace andthrough the third shoot leading from the preheat and prereductionfurnace to the smelting reduction furnace the iron ores preheated andprereduced are transferred and charged into the smelting reductionfurnace.

In addition, leading pipe 33 connects smelting reduction furnace 10 topreheat and prereduction furnace 30, exhaust gas generated from thesmelting reduction furnace is transferred to the preheat andprereduction furnace. From the view point of material, equipment cost,smooth operation and so forth, as the preheat and prereduction furnace,a shaft furnace type vessel with a good heat efficiency or a rotary kilntype vessel useful for cost reduction and easy operation can be employedwithout any difficulty in carrying out the present invention.

Furthermore, hot cyclone 34 for removing dust from exhaust gasgenerating from preheat and prereduction furnace 30 and steam generator35 for obtaining steam by making use of sensible heat of the exhaustgas, each, are set, connected with the preheat and prereduction furnaceas shown in FIG. 1. On the other hand, side tuyeres 25 and bottomtuyeres 26 through which stirring gas is respectively blown in are builtin a side wall and a bottom of smelting reduction furnace 10, each.Furthermore, gas selector valve 38 is set, which controls transferdirection of the exhaust gas coming out of steam generator so as to sendthe exhaust gas to side tuyeres 25 and bottom tuyeres 26 or to exhaustthe same to the outside of the system. It should be noted that the steamgenerator can be alternated to preheat iron ores, using the exhaust gasgenerating from the preheat and prereduced furnace.

Now, a preferred embodiment of a method of the present invention, usingan apparatus constituted as above mentioned will be described.

Before iron ores are charged into a smelting reduction furnace, socalled seed molten metal is already charged therein in ordinaryoperation to make the operation start up smoothly and quickly. When theseed molten metal is not in the smelting reduction furnace, iron scrapsare melted to prepare the seed molten metal.

At the start up of the operation, no slag exists. Slag such as at leastone selected from the group consisting of basic oxygen furnace slag,smelting reduction furnace slag, blast furnace slag and electric furnaceslag is charged together with carbonaceous material and the slag ismelted with combustion of the carbonaceous material by oxygen. Iron oresdo not begin to be charged until an amount of slag reaches 30 kg per 1ton molten metal bath in the smelting reduction furnace. If the amountis less than 30 kg, the slag does not work satisfactorily as a solvent,while if the amount is over 100 kg, the effect of the slag, as asolvent, hits the ceiling. Charging amount of carbonaceous material andblowing amount of oxygen gas are increased in proportion to chargingamount of iron ores. But, unless the start-up slagging is completed,heat efficiency is insufficient or temperature of molten metal does notrise promptly. As a result, abnormal operation such as melting loss of afurnace wall or slopping occurs.

Fluxing material is normally charged in the form of bulky lumps or roughgrains into smelting reduction furnace 10. However, in case that it isnecessary to shorten a time of melting the fluxing material, it iseffective that the powdered fluxing material is blown in together withcarrier gas through oxygen nozzle 21 or side tuyeres 25 and bottomtuyeres 26. 80 kg/T. molten metal, basic oxygen furnace slag was blownin through tuyeres 26 by using Ar or N₂ inert gas, as carrier gas. Onthe other hand, in comparison, as the normal method, 60 kg/T. moltenmetal lime stone and 20 kg/T. molten metal silica were charged throughfirst shoot 13. As to the time period from the start of blowing up tothe start of charging the iron ores, the method using carrier gasrequired 14 minutes, while the normal method required 20 minutes.

After molten slag is thus formed, iron ores, as material of moltenmetal, is charged into smelting reduction furance 10, and the smeltingreduction of the iron ores are started. Normal and basic operationalconditions such as production amount of molten metal, charging amount ofiron ores, blowing amount of oxygen gas are shown in Table 1. Anoperation example described hereinbelow was carried out on the sameconditions as those shown or the like.

                  TABLE 1                                                         ______________________________________                                        Smelting Reduction Preheat and prereduction                                   Furnace            Furnace                                                    ______________________________________                                        Molten Metal                                                                             27.3 (t/Hr) Iron Ores  40.1 (t/Hr)                                 Iron Ores  40.1 (t/Hr) Preheat temp.                                                                            800 (°C.)                            Silica     18 (t/Hr)   Prereduction                                                                             15%                                         Lime Stone 3 (t/Hr)    Ratio                                                  Oxygen Gas                                                                    Decarboniza-                                                                             6650 (Nm.sup.3 /Hr)                                                tion                                                                          Post com-  6650 (Nm.sup.3 /Hr)                                                bustion                                                                       Temp. of   1510 (°C.)                                                  Molten Metal                                                                             4.8% -[C]                                                          ______________________________________                                    

Iron ores, carbonaceous material and lime stone which have been chargedonto molten metal bath 11 and slag layer 12 through first shoot 13 areamply stirred by stirring gas which has been blown in through sidetuyeres 25 and bottom tuyeres 26, thereby the reduction reaction beingpromoted. The stirring gas used for the stirring is process gas which isexhaust gas from steam generator 35, N₂ and/or Ar and is made use ofproperly, depending on operation conditions and material gas.

If a flow amount of stirring gas is excesive, the stirring gas is blownout through the surface of slag layer 12 to make blow-out gas channelingand the affect of the stirring is lost, or at least cannot be expected.Preferable range of the flow amount of the stirring gas blown in throughside tuyeres 25 is 0.3 to 2 Nm³ /min./T. molten metal and the preferablerange of the flow amount of the stiring gas through each of bottomtuyeres 26 is 0.5 to 3 Nm³ /T. molten metal bath.

Now with specific reference to FIG. 3, a typical example of the blow-inof the stirring gas will be described. FIG. 3 shows graphically arelation between Fe content in slag (T,Fe) and amount of stirring gasblown in through bottom tuyeres 26. The Fe content in slag affects ayield of molten metal to be produced and in this respect, it is clearthat the less the Fe content in slag is, the better the yield is. Thisgraphic representation gives a case that the flow amount of the stirringgas blown in through side tuyeres 25 is constantly set as 1.0 Nm³/min./T. molten metal bath. The amount of the Fe content in slag showsthe lowest value, when the blowing amount, of the stirring gas is 3.0Nm³ /min./T. molten metal bath, and the effect of the reduction hits theceiling even if the flow amount is raised more than 3.0 Nm³ /min./T.molten metal bath. On the other hand, if the flow amount is less than0.5 Nm³ /min.T, molten metal bath, there is possibility that sloppingwill occur due to high slag iron content. Consequently, the flow amountof the stirring gas blown in through tuyeres 26 ranges preferably 0.5 to3.0 Nm³ /min./T. molten metal. 2 to 3 Nn³ /min./T. molten metal is morepreferable.

FIG. 4 shows graphically a relation of temperature difference betweenmolten slag and molten metal in the smelting reduction furnace with blowamount of stirring gas through side tuyeres 25. The temperaturedifference indicates a degree of stirring molten metal bath 11 and slaglayer 12. The small difference means that the stirring is amplyperformed. In other words, it is shown that the heat transfer efficiencyto the molten metal is high and consequently, the heat efficiency isgood. The graph gives a case that the flow amount of stirring gasthrough bottom tuyeres 26 is constantly set as 1.8/min./T. molten metalbath. The mentioned temperature difference shows the lowest when theflow amount of the stirring gas is 2.0 Nm³ /min./T. molten metal bath.Even if the flow amount of the stirring gas is increased more than 2.0Nm³ /min./T. molten metal bath, the effect of reducing the temperaturebalance hits the ceiling. On the contrary, if the flow amount is lessthan 0.3 Nm³ /min./T. molten metal bath, the temperature difference isexcessive to allow. Consequently, the flow amount of the stirring gasthrough the side tuyeres ranges preferably 0.3 to 2.0 Nm³ /min./T.molten metal.

Furthermore, the temperature difference is 40° to 60° C. in case thatside tuyeres 25 are not built in the side wall of the smelting reductionfurnace. As shown in FIG. 4, if the stirring gas is blown in throughside tuyeres 25, the temperature difference is 30° C. or less, and theeffect of the stirring is remarkable.

Oxygen gas blown in through decarbonizing nozzles 22 oxidates thecarbonaceous material to supply heat enough to reduce iron ores. Inaddition, oxygen gas is blown in through post combustion nozzles 23, andthis oxygen gas is mainly consumed for burning CO gas generated fromcarbonaceous material by means of oxidation in molten metal bath 11 andslag layer 12.

In FIG. 2, chain line 40 indicates the center axis of oxygen lance 21.An injection angle of α of decarbonising nozzles 22 is determined as 15°or less, based on the conventional BOF oxygen lance, where α is an angleformed by chain line 40 and arrow 28 which is the center axis ofdecarbonizing nozzles 22 and shows a blowing direction. However, aninjection angle of θ of combustion nozzles 23 has an important role ofimproving the heat efficiency of the smelting reduction furnace, whereθis an angle formed by chain line 40 and arrow 29 which is the centeraxis of post combustion nozzles 23 and shows a blowing direction. Apreferable range of this injection angle is determined, based on arelation between the slope angle and the oxidation degree of the exhaustgas generated from the smelting reduction furnace. The relation isgraphically represented in FIG. 5. The degree of the oxidation (OD) isgiven by the following equation:

    OD=(CO.sub.2 +H.sub.2 O)/(CO+CO.sub.2 +H.sub.2 +H.sub.2 O)

As seen from FIG. 5, if the injection angle of θ is less than 30°, CO₂produced by post combustion becomes easy to be reduced by C contained inferrous grains splashed from molten metal bath 11 or C of carbonaceousmaterial and the OD represented by the equation decreases remarkably.Consequently, the post combustion efficiency i.e. the heat efficiency ofthe smelting reduction furnace is lowered. Furthermore, if the injectionangle of θ is over 45°, melting loss of the inner wall of the smeltingreduction furnace caused by oxygen gas coming out of the post combustionis remarkably increased. As this result, the preferable range of theinjection angle of θ is 30 to 45°.

As described in the foregoing, oxygen gas is blown in by means of thepost combustion nozzles. However, if the basicity of slag is notappropriate, the operation is disturbed. With specific reference to FIG.6, the preferable range of the basicity will be described.

FIG. 6 shows a relation between a slag ratio and basicity of slag. Theslag ratio is represented by L_(S) /L_(M). On the Y axis, L_(S)represents thickness of slag layer 12 and L_(M) represents depth ofmolten metal bath 11. The drop of the basicity of slag is caused mainlybecause SiO₂ contained in coal which is charged into the smeltingreduction furnace is melted into slag when the smelting reductionreaction proceeds in the smelting reduction furnace. As seen from FIG.6, slag if the basicity goes down less than 1.2, slag foaming increasesand this causes slopping. Slopping gives unfavourable influence to theoperation of the smelting reduction operation greatly. In order toreduce this phenomenon, fluxing material containing a large amount ofCaO such as lime stone, burnt dolomite or basic oxygen furnace slaggranule is added to slag to increase the basicity of the slag. On theother hand, if the basicity is over 1.8, Fe content in slag increasesand this results in causing a drop of yield of the molten metal. Asmentioned, the basicity can be reduced by SiO₂ contained in coal. Inaddition, in order to reduce the basicity quickly, powder coal is blownin through nozzle 24 of oxygen lance 21 or side tuyeres 25 and/or bottomtuyeres 26. Addition of fluxing material raising the basicity is carriedout similarly. Ordinarily the fluxing material is supplied through firstshoot 13. To control the basicity quickly, powder fluxing material isblown in through nozzle 24 of oxygen lance 21 or side tuyeres 25 and/orbottom tuyeres 26.

What is claimed is
 1. Apparatus for smelting and reducing iron ores,comprising:preheat and prereduction furnace means for preheating andprereducing iron ores, and for generating hot exhaust gas; smeltingreduction furnace means, into which iron ores, carbonaceous material andfluxing material are charged, for smelting and reducing the iron orescharged therein; a top-blow oxygen lance through which oxygen gas isblown in into the smelting reduction furnace means, said oxygen lancehaving decarbonizing nozzles and post combustion nozzles; said smeltingreduction furnace means having bottom tuyeres and side tuyeres forblowing in stirring gas, said bottom tuyeres being built in a bottomportion of said smelting reduction furnace means and said side tuyeresbeing built in a side wall portion of said smelting reduction furnacemeans; hot cyclone means for removing dust from said exhaust gasgenerated from said preheat and prereduction furnace means; means forfeeding exhaust gas, from which dust has been removed, to said bottomand side tuyeres for use as a stirring gas; and steam generator meansfor generating steam from heat of said exhaust gas generated from saidpreheat and prereduction furnace means.
 2. The apparatus of claim 1,wherein:said decarbonizing nozzles of said oxygen lance have a blowingdirection or injection angle of 15° or less relative to a center axis ofthe oxygen lance; and said post combustion nozzles of said oxygen lancehave a blowing direction or injection angle of 30 to 45° relative tosaid center axis of said oxygen lance.
 3. The apparatus of claim 1,wherein said top-blow oxygen lance comprises a single oxygen lance. 4.The apparatus of claim 3, wherein said single oxygen lance is mountedsubstantially at the center of the smelting reduction furnace means.